The reflectivities of the mixed-valent compounds CeSn{sub 3} and CePd{sub 3} have been measured to obtain the optical conductivity as a function of temperature. Both compounds show a renormalization of the low-energy Drude conductivity at low temperatures. In addition, for the low-carrier-density compound CePd{sub 3}, but not for high-carrier-density CeSn{sub 3}, substantial spectral weight lost from the conductivity below 1000 cm{sup {minus}1} reappears in the frequency range 2000{endash}10000 cm{sup {minus}1}, indicating that CePd{sub 3} can be considered a lightly doped Kondo insulator. {copyright} {ital 1996 The American Physical Society.}

Here, the mixed-valent Pbmore » $${}_{3}$$Rh $${}_{7}$$O$${}_{15}$$ undergoes a Verwey-type transition at $${T}_{{\rm{v}}}\approx 180$$ K, below which the development of Rh$${}^{3+}$$ /Rh$${}^{4+}$$ charge order induces an abrupt conductor-to-insulator transition in resistivity. Here we investigate the effect of pressure on the Verwey-type transition of Pb$${}_{3}$$Rh$${}_{7}$$O$${}_{15}$$ by measuring its electrical resistivity under hydrostatic pressures up to 8 GPa with a cubic anvil cell apparatus. We find that the application of high pressure can suppress the Verwey-type transition around 3 GPa, above which a metallic state is realized at temperatures below ~70 K, suggesting the melting of charge order by pressure. Interestingly, the low-temperature metallic region shrinks gradually upon further increasing pressure and disappears completely at P > 7 GPa, which indicates that the charge carriers in Pb$${}_{3}$$Rh$${}_{7}$$O$${}_{15}$$ undergo a reentrant localization under higher pressures. We have constructed a temperature-pressure phase diagram for Pb$${}_{3}$$Rh$${}_{7}$$O$${}_{15}$$ and compared to that of Fe$${}_{3}$$O$${}_{4}$$, showing an archetype Verwey transition.« less

Crystals of the mixed-valent compound (Bi{sub 6}O{sub 5})Rh{sub 8}{sup 3+}Rh{sub 4}{sup 4+}O{sub 24} were grown from a flux. The room temperature conductivity of a crystal was 3 S/cm but decreased smoothly with decreasing temperature to 10{sup -5} S/cm at 25 K. Magnetic susceptibility data indicate a localized moment for Rh{sup 4+}. A Seebeck coefficient at 200 K of +280 {mu}V/K further confirms that this compound is a semiconductor rather than a metal with a partially filled 4d t{sub 2g} band. A structure refinement based on single crystal X-ray diffraction data obtained at 173 and 296 K provided Rh-O distances sufficientlymore » accurate to indicate the nature of the charge ordering between Rh{sup 3+} and Rh{sup 4+}. The large Seebeck coefficient coupled with the high electrical conductivity indicates that this may be a promising low-temperature thermoelectric material. - Graphical abstract: Crystals of the mixed-valent compound (Bi{sub 6}O{sub 5})Rh{sub 8}{sup 3+}Rh{sub 4}{sup 4+}O{sub 24} show semiconducting properties, and magnetic susceptibility data indicate a localized moment for Rh{sup 4+}. Structure refinements at 173 and 296 K show partial charge ordering between Rh{sup 3+} and Rh{sup 4+}.« less